All of the following materials are radiopaque in nature except:

All given options are metallic (radiopaque) in nature except composite resins. Generally, non-metallic radiopaque markers (gutta-percha, composite resin) are used in MDCT imaging and CBCT imaging studies to minimize the effect of image artifacts produced by metal markers.   Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 637